In optical coherence tomography OCT systems the lateral resolution (x and y) is defined by the numeral aperture (NA) of the optical system used. The axial resolution (z), however, is calculated from an interference pattern and as a rule is much greater than the depth of field of the imaging, which in turn depends on the numerical aperture, more precisely is proportional to 1/NA2. In the usually used Fourier domain OCT, which uses a broadband or wavelength-adjustable radiation source, the depth resolution is inversely proportional to the spectral bandwidth, more precisely proportional to λ2/Δλ, wherein λ is the average wavelength and Δλ is the bandwidth. Optical coherence tomography (OCT) is an established method for imaging the eye in ophthalmology. It makes a three-dimensional imaging possible, which is very useful for the diagnosis of eye diseases and the progression thereof. To be named in particular here are diseases of the retina, such as glaucoma or age-related macular degeneration.
To measure the retina of the human eye both a high lateral resolution and a high axial resolution are needed. At the same time the detectable and thus illuminated volume is to be as large as possible in terms of depth (along the optical axis); this requires a small numerical aperture (NA) of the optical system. The lateral resolution requires a large numerical aperture. Thus, ultimately, in the state of the art, the extent of the area accessible in terms of depth and the lateral resolution are linked to each other via the numerical aperture of the optical system and cannot be set independently of each other.
An OCT based imaging method is known from US 2014/0028974 A1. A line is projected through an imaging system onto a sample. The backscattered radiation is combined in an interfering manner with reference radiation and guided to a detector, wherein a confocal filtering is carried out in one direction. An astigmatic optical system is used for this. The depth resolution is obtained by optical coherence tomography. In the case of a spectroscopic analysis of the radiation, a two-dimensional detector is used one dimension of which is used for the confocal filtering with respect to the illuminated line area and the other dimension of which resolves the spectral information. Lateral resolution and accessible depth area are also linked in the approach according to US 2014/0028974 A1.
WO 2014/0179465 A1 describes an OCT which operates in the spectral domain, thus analyses the interference of radiations with a spectrometer. The light source emits a bundle of light which consists of a plurality of parallel individual bundles which are imaged onto the sample through the objective lens. A reference arm also guides several parallel individual bundles, with the result that in the end each individual bundle is guided through the device according to the OCT measurement principle and also analysed on the spectrometer. This device is very complicated to align.
In a scanning OCT system the accessible diameter of the pupil of the eye is usually between 1 mm and 1.5 mm. This results in a lateral resolution of approximately 15 μm and an area detectable in terms of depth with an extent of 3 mm. A better lateral resolution would be achieved with a higher numerical aperture of the optical system. However, at the same time, the depth-detectable area would thus be reduced. In addition, aberrations increase with the numerical aperture. In known OCT systems which use a diameter of up to 1.5 mm in the pupil of the eye astigmatism and coma increase for larger pupils even if the defocusing is disregarded as a higher-order aberration. A diffraction-limited resolution can therefore not be achieved.
For particular applications, in particular for the diagnosis of age-related macular degeneration, a high lateral resolution is desired. In order to diagnose an early stage of this disease, a lateral resolution of approximately 5 μm is needed. At the same time, a depth measurement area that can be sampled of approximately 3 mm is required, as it is assumed that age-related macular degeneration is accompanied by blood vessel growth in deeper layers of tissue. In order to detect such vessels, a good signal-to-noise ratio is additionally needed.